Electrochemical fuel cell systems typically include a stack of fuel cells having electrolyte membranes between catalytic anodes and cathodes. During normal operation, fuel flows through the anodes and is catalyzed into useful electrons and byproduct protons, which pass through the membranes to react with an oxidant flowing through the cathodes to produce byproduct water. During shutdown, oxidant flow is terminated but fuel flow is continued according to a pressure setpoint to ensure consumption of all residual oxidant, and the stack is discharged to avoid undesirable fuel cell voltages.
But stack discharge induces rapid reaction of the residual oxidant and the flowing fuel, causing anode pressure to subcede the setpoint. A controller responds by increasing fuel injection, but the residual oxidant is consumed so rapidly that the rate of fuel consumption collapses by the time the additional fuel is actually injected, thereby leading to an excess of anode pressure. In other words, controller lag results in anode pressure overshooting, and any tuning of controller gain to correct overshoot involves unacceptable response time. Accordingly, anode pressure is difficult to maintain at the shutdown setpoint, and resulting anode pressure fluctuations may damage the membranes and cause negative cell voltages on the stack.